The present invention relates to a developing device and an image forming apparatus which develop a latent image on an image carrying member with developer containing toner and carrier.
Conventionally, as a developing method for an electrostatic latent image formed on an image carrying member in an image forming apparatus employing an electrophotographying technique, there have been well known the one component type developing method using only toner as developer and the two component type developing method using toner and carrier.
Generally, in the one component type developing method, toner is electrically charged when passing a regulating section formed with a toner carrying member and a regulating plate being pressed onto the toner carrying member, and the toner forms a desired toner thin layer and develops an electrostatic latent image. For this reason, since the developing is conducted on the condition where the toner carrying member and the image carrying member are arranged to be adjacent to each other, the reproducibility of dot becomes excellent. Further, when a uniform toner thin layer is formed, a uniform image may be easily obtained without unevenness of an image which is caused by magnetic brush and observed in the two component type developing method. Further, it has been considered that the one component type developing method is advantageous in respect of making an apparatus simpler, smaller, and with lower cost. However, on the other hand, the quality of the surface of toner alters with a strong stress in a regulating section and the electric charge receptiveness of the toner decreases. Then, a toner regulating member and the surface of a toner carrying member are polluted with adhesion of toner or an external additive, and a electrically charging ability for toner reduces, thereby causing a problem of fogging or soil at an inside of an apparatus by poorly charged toner. As a result, there may be a problem that the life of a developing device becomes shorter.
On the other hand, in the two component type developing method, since toner is electrically charged with triboelectrification when the toner is mixed with carrier, stress applied to toner is smaller. Therefore, the two component type developing method is advantageous in respect of degradation of toner. Furthermore, since the carrier being a charge providing member for toner has a larger surface area, the two component type developing method relatively hardly causes pollution by toner or an external additive, and is advantageous in respect of extension of life-span.
However, when two component developer is used, there may be no change in that pollution on the surface of carrier is caused by toner or an external additive. The usage over a long period of time causes lowering of an electrically charged amount of toner, thereby further causing problems, such as fog and toner scattering. Accordingly, it can be said that its life is by no means enough, and then extension of life-span is desired more.
As a method of extending the life-span of two component developer, Patent documents 1 discloses a developing device to replenish a small amount of carrier independently or together with toner for each time; and to discharge deteriorated developer having a decreased electrically chargeable ability in accordance with the replenishment so as to replace carrier; thereby suppressing increase of the ratio of deteriorated carrier. In this device, since carrier is replaced, it becomes possible to suppress decrease of an electrically charged amount of toner due to the deterioration of carrier at a predetermined level, and this technique is advantageous to extension of life-span.
Moreover, Patent documents 2 discloses two component developer composed of carrier and toner externally added with particles having an electrically chargeable property with a reverse polarity against the electrically charged polarity of the toner and a developing method using this two component developer. In this developing method, the reverse polarity chargeable particles are added with a target for the action as abrasive powder and spacer particles, and it is shown that the effect to remove spent materials on the surface of carrier further causes an effect to suppress deterioration. Furthermore, it is reported that in the cleaning section of an image carrying member, there is an improvement in cleaning ability and an effects in polishing for an image carrying member.
Moreover, Patent document 3 discloses a so-called hybrid type developing method in which only toner of two component developers is made to be carried on a toner carrying member arranged to oppose to an image carrying member and a latent image on the image carrying member is developed with the toner. The hybrid type developing method has the following features which are not in the ordinary two component developing method. Unevenness of an image caused by a magnetic brush does not occur, and it is excellent in dot reproducibility or evenness of an image. Since a magnetic brush is not brought in direct contact with an image carrying member, shift (carrier consumption) of carrier to an image carrying member does not take place. Further, with a structure having a recovery developer carrying member for recovering toner from a toner carrying member after development, a counter measure is taken for a problem of an image memory phenomenon in which a previous image pattern is reflected to the following image. In the hybrid type developing method, since electrically charging for toner is performed by triboelectrification with carrier, maintenance of the charge providing ability of carrier is important in order to stabilize the electrically-charged ability of toner and to maintain a good image quality over a long period of time.    [Patent documents 1] Japanese Patent Unexamined Publication No. 59-100471 Official gazette    [Patent documents 2] Japanese Patent Unexamined Publication No. 2003-215855 Official gazette    [Patent documents 3] Japanese Patent Unexamined Publication No. 10-340003 Official gazette
However, in Patent documents 1, there are following problems: It is necessary to provide a device to recover the discharged carrier. Since carrier becomes consumable goods, there are problems in cost and an environmental aspect. Moreover, until old and new ratio of carrier becomes stabilized, it is necessary to repeat printing of predetermined amount, and it is not necessarily possible to maintain an early characteristic. Further, in Patent document 2, there is a problem that since the consumption of toner and reverse polarity chargeable particles varies in accordance with imaging area ratios, when an imaging area ratio is small, the consumption of the reverse polarity chargeable particles adhering to a non-imaging area having a large area becomes superfluous, and then an effect to suppress deterioration of carrier exiting in a developing device decreases. Furthermore, in the hybrid type developing method of Patent documents 3, there was a problem that the surface of carrier was polluted with toner, a post treatment agent and so on with increase of the number of printed sheets, resulting in that the charge providing ability of the carrier decreases.